In astrophysics, photodissociation regions (or photon-dominated regions, PDRs) are predominantly neutral regions of the interstellar medium in which far ultraviolet photons strongly influence the gas chemistry and act as the most important source of heat.[1] They occur in any region of interstellar gas that is dense and cold enough to remain neutral, but that has too low a column density to prevent the penetration of far-UV photons from distant, massive stars. A typical and well-studied example is the gas at the boundary of a giant molecular cloud.[1] PDRs are also associated with HII regions, reflection nebulae, active galactic nuclei, and Planetary nebulae.[2] All the atomic gas and most of the molecular gas in the galaxy is found in PDRs.[3]


The study of photodissociation regions began from early observations of the star-forming regions Orion A and M17 which showed neutral areas bright in infrared radiation lying outside ionised HII regions.[3]


  1. ^ a b Hollenbach, D.J.; Tielens, A.G.G.M. (1999). "Photodissociation regions in the interstellar medium of galaxies". Reviews of Modern Physics. 71 (1): 173–230. Bibcode:1999RvMP...71..173H. doi:10.1103/RevModPhys.71.173.
  2. ^ Tielens, A.G.G.M. (1993). "Photodissociation regions and planetary nebulae". Planetary Nebulae: Proceedings of the 155 Symposium of the International Astronomical Union. 155: 155–162. Bibcode:1993IAUS..155..155T. doi:10.1017/S0074180900170330.
  3. ^ a b Hollenbach, D. J.; Tielens, A. G. G. M. (1997). "Dense photodissociation regions". Annual Review of Astronomy and Astrophysics. 35: 179–215. Bibcode:1997ARA&A..35..179H. doi:10.1146/annurev.astro.35.1.179.